Conventionally, a three-way catalyst is disposed in an exhaust passage of an internal combustion engine in order to purify an exhaust gas discharged from the engine. As is well known, the three-way catalyst has an “oxygen storage function” for storage or release oxygen depending on components included in a gas flowing into the three-way catalyst. Hereinafter, the three-way catalyst is also simply referred to as a “catalyst”, and the gas flowing into the catalyst may also be referred to as a “catalyst inflow gas”.
A conventional air-fuel ratio control apparatus (conventional apparatus) comprises a downstream air-fuel ratio sensor disposed downstream of the catalyst in the exhaust passage of the engine. The conventional apparatus obtains a “base fuel injection amount to have an air-fuel ratio of a mixture supplied to the engine coincide with (becomes equal to) a stoichiometric air-fuel ratio” based on an air amount introduced into a cylinder, and corrects the base fuel injection amount base on at least an output value of the downstream air-fuel ratio sensor.
More specifically, the downstream air-fuel ratio sensor is an oxygen concentration sensor of an oxygen concentration cell type, and outputs an output value Voxs (refer to FIG. 3). The output value Voxs of the downstream air-fuel ratio sensor becomes (reaches) a maximum output value Vmax when an air-fuel ratio of a gas flowing out from the catalyst (hereinafter, also referred to as a “catalyst outflow gas”) is smaller than the stoichiometric air-fuel ratio (i.e., richer than the stoichiometric air-fuel ratio), that is, when an excessive amount of the oxygen is not included in the catalyst outflow gas. The “case in which the excessive amount of oxygen is not included in the catalyst outflow gas” means a case in which the oxygen is insufficient, and unburnt substances remain as a result of a combination of “the unburnt substances and the oxygen” both included in the catalyst outflow gas. In other words, the “case in which the excessive amount of the oxygen is not included in the catalyst outflow gas” means a case in which an amount of the oxygen included in the catalyst outflow gas is smaller than an amount necessary to oxidize all of the unburnt substances included in the catalyst outflow gas.
Further, the output value Voxs of the downstream air-fuel ratio sensor becomes (reaches) a minimum output value Vmin when the air-fuel ratio of the catalyst outflow gas is larger than the stoichiometric air-fuel ratio (i.e., leaner than the stoichiometric air-fuel ratio), that is, when an excessive amount of the oxygen is included in the catalyst outflow gas. The “case in which the excessive amount of the oxygen is included in the catalyst outflow gas” means a case in which the unburnt substances disappears (are consumed), and the oxygen remains as a result of the combination of “the unburnt substances and the oxygen” both included in the catalyst outflow gas. In other words, the “case in which the excessive amount of the oxygen is included in the catalyst outflow gas” means a case in which an amount of the oxygen included in the catalyst outflow gas is larger than the amount necessary to oxidize all of the unburnt substances included in the catalyst outflow gas.
In this manner, the output value becomes the minimum output value Vmin when the excessive amount of the oxygen is included in the catalyst outflow gas, and the output value becomes the maximum output value Vmax when the excessive amount of the oxygen is not included in the catalyst outflow gas. Accordingly, it is assumed that the air-fuel ratio of the catalyst outflow gas coincides with the stoichiometric air-fuel ratio, when the output value Voxs is equal to a “value Vmid which is a middle value (mid-value, mean value) between the maximum output value Vmax and the minimum output value Vmin (i.e., the middle value Vmid=(Vmax+Vmin)/2)”.
In view of the above, the conventional apparatus calculates an air-fuel ratio feedback amount according to a proportional-integral control (PI control) and the like in such a manner that the output value Voxs of the downstream air-fuel ratio sensor becomes equal to (coincides with) a “target downstream-side value Voxsref which is set at a value corresponding to the stoichiometric air-fuel ratio (that is, the middle value Vmid)”. The air-fuel ratio feedback amount is also referred to as a “sub feedback control amount”, for convenience. The conventional apparatus corrects the base fuel injection amount based on the sub feedback amount to thereby control the air-fuel ratio of the mixture supplied to the engine, so that it controls the air-fuel ratio of the catalyst inflow gas (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-171982).